Javascript - Validar email

<script>
    function validaEmail(valor) { 
        var reg = /^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/;               
        return reg.test(valor); //retorna true ou false
    }
    alert(validaEmail('email@email.com')); // retorna true
    alert(validaEmail('email la la la')); // retorna false
</script>

Fonte: http://www.white-hat-web-design.co.uk/blog/javascript-validation/